Froth system for continuous manufacture of polyurethane foam slab-stocks

ABSTRACT

A process and a system for the continuous manufacture of polymeric foams. Reactive chemical components and additives comprising a low boiling blowing agent are mixed under pressure; the mixture is then frothed before chemical reaction takes place by feeding the mixture through a pressure equalizing and frothing device having a pressure-drop zone opening into a frothing cavity having an output aperture to discharge the froth onto a moving substrate.

This is a division of application Ser. No. 08/271,918, filed Jul. 8,1994.

FIELD OF THE PRESENT INVENTION

The present invention relates to the production of polymeric foams byusing the frothing technique, and more particularly is directed to aprocess and a system for the continuous production of flexible and rigidslab-stocks, normally used for providing panelling, soft cushioning, andthe like.

BACKGROUND OF THE PRESENT INVENTION

Polymeric foams in particular polyurethane foams are well known. Ingeneral their preparation requires the mixing of reactive chemicalcomponents, such as a polyol and an isocyanate, in the presence ofnormally used additives such as a suitable catalyst, a surfactant orcell control agent, and water which chemically reacts with theisocyanate to produce the carbon dioxide for blowing the foam.

In the continuous production of flexible foams and particularly in theproduction of flexible foams in slab-stocks, as currently practised onconventional machines, it is common practice to spread or pour a thinlayer of the mixture in a liquid state onto a moving sheet substrateprovided on a slightly sloped conveyor and then the foam is allowed torise freely, due to reaction between the chemical components, until thetotal expansion of the foam is obtained. The foam is then allowed tocure and thereafter is cross-sawn into blocks. Convention process andapparatus are described, for example, in U.S. Pat. Nos. 3,325,823 and4,492.664.

In order to avoid a situation where the liquid mix underruning the foam,and to allow the production of uniform blocks, use of a small slope, andhigh speed for the conveyor and high chemical out put are usuallyrequired. This results in costly and large space consuming machines, aswell as in an excessively high production rate and very large scaleplants.

In an attempt to partially remedy the problems and disadvantagesinvolved by a conventional process, U.S. Pat. No. 3,786,122 suggests analternative foaming procedure, in which liquid reactants are mixed andintroduced in the liquid state, at the bottom of a pre-foaming trough.This allows the mixture to expand upwardly causing the pre-expandedmixture to flow out of the vessel, on a channel-shaped sheet materialtravelling on a conveyor device. Although this process eliminates theuse of reciprocating mixing heads in the production of a continuous slabof a polymeric foam, nevertheless some problems arise due to "build-up"of the foam in the trough which causes a progressive narrowing orreduction of the useful volume of the trough and consequently areduction in the residence time in the through. The partly expanded foamis still of a relatively high density and low viscosity and limits theslope angle that can be used for the foaming conveyor. Consequently, therisk of foam underrunning, therefore, still exists.

Although the main object of the U.S. Pat. No. 3,786,122 was to employ aconveyor shorter in length, running at a speed slower than the conveyorin a conventional machine, the fact that the mixture emerging from thevessel is still in a liquid state practically prevents the speed and thelength of the conveyor to be reduced to any substantial extent.Therefore the resulting slab-stock foaming machine was large and stillrequired large spaces. A Publication PU Handbook G. HOERTEL Ed. CarlHauser Verlang--1985 pages 162-168 describes a further variation of thepouring technique in the attempt to achieve an uniform lay-down of amixture of polyurethane components by a fixed mixhead. This mixturebeing discussed by Hoertel is not a froth nor does blowing occur.According to Hoertel the mixture is spread onto the whole width of theconveyor by the use of a distribution bar through which the mixture isdelivered across a broad front. In the same manner the resulting flow issimilar to that resulting from the trough used in the U.S. Pat. No.3,786,122; depending on the volume of the distribution bar and thechemical reactivity of the foam system, the mixture is delivered onto asubstrate as a clear (no reaction) or an already creamy (reactionstarted) liquid. Consequently, density and viscosity of the mixturestill depend on the volume of the distribution bar in which chemicalfoaming takes place. Therefore, the distribution bar according toHoertel does not substantially differ from the trough system of U.S.Pat. No. 3,786,122.

The frothing process is a well known technique in polyurethanetechnology but not in the production of slab stock foam. When frothing,a non-reactive inert gas, or blowing agent, is mixed under pressure in aliquid state or in solution with the polyurethane chemical components ina mixer. The pressure is subsequently released causing the frothing orpre-expansion. The vaporization of the blowing agent causes the cells togrow and to foam the liquid reaction mixture which cures to form anelastomer.

Typical blowing agents are the various chlorofluorocarbons (CFC),however certain environmental problems are associated with the use ofCFC materials. Therefore, many attempts have been made to produce foamedpolyurethane materials, by frothing with carbon dioxide (CO₂).

Carbon dioxide (CO₂) as non-reactive blowing agent, in the frothingtechnique, has been suggested for example by U.S. Pat. Nos. 3,184,419and 5,120,770.

According to these two patents, the reaction mixture is subjected to apressure during mixing, to maintain the blowing agent in the liquidstate. Thereafter the mixture is ejected at atmospheric pressure causinga turbulent vaporisation of the blowing agent. Therefore, while thefroth technique and the use of an inert blowing agent incorporated in aliquid state into the reaction mixture, allows the manufacture of a foamof reduced density, nevertheless the cell structure is of veryinconsistent quality due to irregular shaped and oversized cells orbubbles being present.

Although frothing with inert gas, in particular CO₂, is a well knownpotential technique, up to now no successfully practicable frothingprocess and system have been suggested or discovered for use with and inslab stock foam production.

In an attempt to solve the problem of slab stock production without useof chlorofluorocarbon blowing agents, and by using the frothingtechniques, it has been now discovered that the suitable release of themixture under pressure must take place under controlled conditions. Useof controlled conditions in the production of polymeric foams positivelyinfluences the growth of the cells during initial frothing expansion ofthe mixture, which is of importance in the production of slab stock.

Presently, the need for a new foaming process and system in thecontinuous manufacturing of flexible slab-stock or other continuous foamproduction lines, in which the frothing technique and a non-reactiveblowing agent could be practically usable, still exists.

OBJECTS AND SUMMARY OF THE PRESENT INVENTION

It is an object of the present invention to provide a foaming process asreferred above, by which it is possible to advantageously use anon-reactive liquid blowing agent, preferably carbon dioxide, to froth,without negatively affecting the cellular structure of the foam, toprovide an appropriate bubble free commercial product.

Another object of the present invention is to provide a process and asystem as referred to above, for continuous foaming, in the productionof flexible or rigid slab-stocks, in which the frothing of the mixturemay be usefully performed under controlled conditions allowing thespreading of the mixture on a moving substrate in the form of a highviscous froth, without involving turbulent vaporisation of the blowingagent. This also permits production of a low density foam on the orderof 14 kg/m³ (0.87 pcf).

A further object of the invention is to provide a foaming process andsystem as referred to above, which make possible the use of a low outputplant running at very low speeds thereby permitting foam lines that aremuch shorter in length, in comparison to conventional or known machinesor production lines or plants, avoiding limitations of the conventionalsystems while maintaining the advantageous features thereof. This alsoresults in a dramatic reduction of exhaust, from a shortened lineoperating at slower conditions, so that the volume/hour of exhaust fumesthat require scrubbing or other cleaning effort prior to release issimilarly reduced. Equally important is the lessening of the amount offactory air that needs to be conditioned thereby reducing energy costsassociated with slab stock production.

The main objectives of the present invention are therefore theelimination of cloro-fluoro carbons (CFC) and volatile organic compounds(VOC) from the formulation and substitution thereof with a lessexpensive component, as well as production of soft, low density foam,with a very homogeneous cell structure, free from large bubbles,pinholes and visible defects.

Contrary to general trends of conventional prior art, the inventionresides in mixing under pressure a reactive blend of polymeric chemicalcomponents and an inert, low boiling blowing agent, followed by frothingthe mixture, prier to the start of any reaction, underpressure-controlled conditions through a pressure equalising andfrothing device having an elongated pressure-drop zone extendingcross-wise to the moving direction of a substrate. Then the frothingmixture is restrained along a frothing passageway or cavity. Thereafter,reaction of the frothed mixture begins. That frothing passageway orcavity preferably has an output aperture of larger area relative to thearea of the aperture or outlet of the pressure-drop zone. According tothe invention, the pressure during mixing ranges preferably from 5 toabout 18 bar.

The present invention is unique in the use of a frothed form of themixed chemical ingredients comprising the polymeric foam to form slabstock. The present invention is unique too in that it uses anenvironmentally safe blowing agent to develop the froth in themanufacture of slab-stock foam. Such foam slab stock can he eitherflexible or rigid foam. The invention employs a specially designeddischarge or lay-down head, called a gate bar, that uniquely aids themixed foam ingredients to froth. This frothing occurs in a verycontrolled way by use of an elongated pressure-drop zone, as part of thegate bar, and then by having the mixed, frothing material pass through afrothing passageway or cavity wherein gaseous blowing agent is graduallyreleased into the frothing mixture prior to discharge onto a movingsubstrate or conveyor. The discharge head assures the frothed materialis distributed across the width of the machine, either across asubstantial portion of the slab stock machine or across a desiredportion thereof. The slab stock machine can include a complete line orplant in which the frothed foam will be permitted to chemically react,fully rise, cure and then be cut into desired pieces.

From experimental tests has been also noted that critical factors forproducing a large foam block are the equilibrium between foam profileangle, metering machine's throughout, conveyor speed and formulationcharacteristics, such as viscosity build-up reactivity, etc. Accordingto present invention, because the mixture, when reaction begins, isalready viscous and supports a steep rise angle, the foam's rising angleis no longer a limitation on the process conditions. The froth beingdischarged from the frothing cavity is a homogeneous pre-expandingmixture with a sufficiently high viscosity to avoid rise angle problemsassociated with prior production equipment and flexible slab stocklines. The viscosity is enough to sustain the production of high blocks,that have fully reacted, even at very slow speeds with steep fall plateangles. This condition is accomplished by controlling the expansionphase of the mixture, after the mix head, and allows for the progressiverelease of the blowing agent in the reacting mass. Accordingly, the fourcritical factors can be varied to achieve the desired density ratherthan having to be linked to a rigid set of parameters as was the casewith prior foam processes. The speed and size of a line or plant can betailored to the needs of the foam manufacturer with speeds from 1 to 5meters per minute, and lengths as short as 20 meters or less, ratherthan the more conventional length of about 100 meters. This also permitsa smaller volume per hour of exhaust to be dealt with and removed orscrubbed, simpler metering and plant fabrication, foams made with CO₂, asmaller volume of air to be conditioned, and very low densities down toabout 14 kg/m³ or less.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and preferred embodiments thereof, will be now describedin great detail, with reference to the accompanying drawings, in which:

FIG. 1 shows a system which operates according to the frothing andfoaming process of the present invention;

FIG. 2 is an enlarged view of a frothing device embodying the featuresof the present invention;

FIG. 3 shows a second system embodying the features of the presentinvention.

FIG. 4 shows a further system embodying a modified form of a frothingdevice;

FIG. 5 is an enlarged view of the frothing device of previous FIG. 4;

FIG. 6 shows an enlarged detail of a further frothing device;

FIG. 7 shows another embodiment of the frothing device according to theinvention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The essential features of the invention are now described in greatdetail with reference first to the appended FIGS. 1 and 2. As shown, theapparatus for performing the process according to the invention, isprovided with two side walls 4 arranged to be perpendicular to theconveyor 1. Continuous side papers 3 are arranged to run and move alongthe inside of each side wall 4 in the production direction P (see thearrow) of the foam. A continuous paper sheet 2 slightly wider than thedistance between the side walls 4, is fed onto the conveyor 1 to providea foaming path. The excess layer at each side is turned up so as to forma seal against the bottom edge of the side papers 3. The bottom paper 2and the side papers 3 effectively form a continuous open-topped trough,which could be, for example, about two meters wide by 1 meter high. Theconveyor 1 may be substantially arranged with a horizontal dispositionor, alternatively, it may be formed with a small angle such as sixdegrees.

The apparatus furthermore comprises a mixer 11 having an outlet 12positioned at a level above the conveyor 1, for feeding a mixture ofreactive chemical components to a frothing device, generally indicatedat 13, and schematically shown in the detail of FIGS. 2 and 5.

A pressure gauge 12' is provided at the outlet spout 12 of the mixingdevice 11 to indicate the pressure of mixed chemicals which leave themixing chamber through the outlet 12. Tanks 5, 6 and 7 are provided tohold the foam forming chemical components such as a polyol and anisocyanate as well as conventionally used chemical additives comprisinga low boiling blowing agent such as CO₂. From each of the tanks, thechemical components and blowing agent are fed to the mixer 11 by pipesand respective metering pumps 8, 9 and 10.

According to the embodiment as shown in FIG. 2, the outlet spout 12 ofthe mixer 11 is connected by tubing 18 to the mixture frothing device 13transversally extending across the conveyor 1. The frothing device 13includes drop-pressure generating means comprising an elongateddrop-pressure zone in the form of an elongated slot 17. It should beunderstood that the purpose of the elongated pressure drop zone is toprovide a back-pressure and allow the mixing under pressure of chemicalcomponents into the mixer 11 as well as an equalization of the pressurein the same frothing device, before the pressure reduction, to preventturbulent evaporation of the blowing agent during frothing of themixture. At the same time the frothing device 13 allows frothing of themixture under restrained and pressure controlled conditions. Thefrothing device 13 also assures that the resulting froth is smoothlydelivered onto the substrate 2, 3 while the same mixture is flowing inthe moving direction of the conveyor device.

As referred above, and shown in the example of FIG. 2, the pressureequalizing and frothing device 13 can, for example, comprise a tubularmember defining an elongated pressure equalizing chamber 21 having alongitudinal axis. Chamber 21 is connected at one or more feedingpoints, to the mixer 11 by at least one tubing arrangement 18. Thepressure equalizing or tubular chamber 21 is provided with apressure-drop zone, for example, in the form of the elongated slot 17which extends longitudinally along one side of chamber 21. However, thepressure drop zone could be formed from other suitably shapedpressure-drop apertures for flowing the mixture before frothing. As isshown in FIGS. 6 and 7, the discharge or gate bar could include a seriesof apertures of circular, oblong or rectangular shape, or a series ofelongated, but shorter, slots so long as the controlled conditions wereproduced. The slot or more precisely said pressure-drop zone 17 has arestricted cross-sectional area sufficient to cause a pressure reductionin the mixture emerging from the chamber 21 during frothing, and acorresponding back pressure in the mixing device 11 for the purposementioned above.

Froth control, to obtain the controlled conditions that are desiredduring lay down of the froth, is accomplished by use of an enlargedcavity, such as is shown at 19 having an outlet aperture 20 from whichthe frothing mixture is delivered onto the moving substrate.

The enlarged frothing cavity 19 has a suitable configuration that willprovide flow control over the frothing mixture and in particular thecross-sectional area of the output aperture 20 should be greater thanthe cross-sectional area of pressure-drop zone, such as the elongatedslot 17, which provides communication between the pressure equalizingchamber 21 and the frothing cavity 19. The sectional area of the outputaperture 20 should range from ten to hundreds of times, or more, thesectional area of the pressure-drop zone in the form of the elongatedslot 17.

In the embodiment shown in FIG. 1, the frothing device 13 is deliveringthe frothed mixture onto the substrate 2/3 which is moving on theconveyor 1 along a substantially horizontal path. The conveyor may beoperated at a rate of about 1 to 5 meters per minute, so that the fullblock height may be achieved within a range of about 1 to 8 meters fromthe deposit point of the froth.

The above example of FIG. 1 refers to a system in which the foam riseupwardly from the bottom of the conveyor.

The example of FIG. 3 refers to a different system in which the foam isallowed to froth onto a downwardly sloping path which allows the lengthof the machine to be greatly reduced. According to the embodiment, thesloping angle of the conveyor, onto which the frothing mixture isdischarged, is not critical due to the high viscosity of the frothemerging from the frothing cavity. Therefore, underrunning does notoccur.

The continuous slab-stock machine of FIG. 3 continues to employ a mainconveyor 1, together with the associated side walls 4 and side andbottom papers 2 and 3, and that conveyor 1 is in a horizontal position.

This embodiment also includes a second driven conveyor 25 inclined at anangle α, for example of about 30 degrees to the horizontal, andpositioned between the side walls and side papers in such a manner thatits lower end is immediately above the main conveyor 1. The pressureequalizing and frothing device 13, such as the one described in Exampleof FIG. 1 has been positioned immediately above the upper surface of theconveyor 25 at its highest point. As an alternative to the conveyor 25,it is possible to use a slide surface, such as a fall-plate forestablishing the downwardly directed path. The angle α preferably canvary from about 10° to about 40° with a range of about 25° to about 30°being the preferred angle.

The bottom paper 2 in this case, runs onto and down the upper surface ofthe slanted conveyor 25 and then onto and along the main conveyor 1.

A second continuous paper 22 runs under a roller 23 so that it justclears the upper surface of the frothing cavity 19 and in such a mannerthat the paper 22 rests on and runs with the top surface of theexpanding foam 16.

Optional pressure plates 24 are fitted so as to rest on the top side ofthe paper 22, if required, to assist in shaping of the expanding foam.

Comparative tests have been conducted in the continuous manufacture ofslab-stocks by means of a process and a system according to presentinvention, in comparison with a conventional system.

EXAMPLES Example 1

According to this comparative example, a conventional continuousslab-stock machine as described in FIG. 1 was used except for thefrothing device 13 which had been eliminated. A blend of chemicals,identified as Blend A in the following Table 1, was made and introducedinto tank 5. The 80:20 TDI, Part B, was introduced into tank 6. Tank 7held stannous octoate catalyst, Part C.

The pumps 8, 9 and 10 were set to give the outputs as defined in Table 1for Parts A, B and C respectively. Conveyor 1 was set to run at a speedof 5 meters per minute.

The foam-forming chemicals, coming from the mixer, were allowed to pourfrom the mixing head outlet 12 directly onto the paper 2. The reading onthe pressure gauge 12' was zero.

Expansion of the foam 16 started at a point along the length of theconveyor 1 about 0.8 meters from the mixing head 11. This represented atime of about 12 seconds from mixing.

The foam was fully expanded by a point about 8 meters from the mixinghead 11. This represented a time of about 105 seconds from mixing. Theheight of the foam block after it had cured was 0.8 meters. The densityof a sample of foam cut from the block was 21.5 kg/m³. The cellstructure of the foam was of a consistent quality with few irregularshaped cells, oversized cells or voids being present.

                  TABLE 1                                                         ______________________________________                                                                   Examples Example                                                              1-3      4                                         ______________________________________                                                          Parts    Output   Output                                                      by       in       in                                                          weight   kg/min   kg/min                                    ______________________________________                                        PART A                     120      24                                        Polyether Polyol, 3500 mw                                                                       100.00                                                      Water             4.50                                                        Amine catalyst, Niax A1                                                                         0.10                                                        Silicone, Tegostab B2370                                                                        1.20                                                        PART B                     62.4     12.5                                      80:20 TDI         55.2                                                        PART C                     0.25     0.05                                      The catalyst, stannous octoage                                                                  0.22                                                        ______________________________________                                    

NIAX is a Trade mark of Union Carbide

TEGOSTAB is a Trade Mark of TH Goldschmidt

(blowing agent having comparatively low cost)

Example 2

According to this comparative example, the same slab-stock foam machineas used in Example 1 was utilized. In this case the blend of chemicalsdesignated Part A in Table 1, which was in the tank 5 was saturated withcarbon dioxide gas at pressure, by addition of a quantity of liquidcarbon dioxide. Sufficient liquid carbon dioxide was added to achieve apressure in tank 5, as indicated by the pressure gauge 14, of 6 bar.

The slab-stock foam machine was operated in the same manner as describedin Example 1.

This time, the chemicals left the mixing head outlet tube 12 in the formof a turbulent froth. It was also noticed that large bubbles of gas wereforming in the foam after being deposited on the bottom paper 2. Thepressure indicated by the pressure gauge 12' on the outlet 12, was zero.

The foam expansion was completed at a distance of about 7 meters fromthe mixing head 11.

When the foam block had cured, it was found to have a height of 0.9meters, indicating that more expansion had occurred than in Example 1.

The density of a sample of foam cut from the block was found to be 19kg/m³.

The cell structure of the foam was inconsistent, there being present inthe foam large ovaloid voids or pin holes up to 30 millimeters high byup to 10 millimeters diameter. The presence of these irregularities inthe cell structure made the foam commercially unacceptable.

Example 3

According to the invention the same slab-stock frothing foam machine asused in Example 1 and 2 was employed, except that a frothing device 13according to the invention, was fitted to the mixing head outlet (12) soas to equalize the pressure of the foam forming chemicals across thewidth of the conveyor (1) before allowing them to froth by reducingtheir pressure on passing through pressure drop zone 17 of the pressureequalising chamber 21.

The pressure equalizing, frothing device illustrated diagrammatically inFIG. 2, consisted of a 1.9 meter length of steel tube 21 of internaldiameter 30 millimeters. Along the length of the pressure equalizingchamber 21, in the form of a steel tube was cut a slot 17 of a height of0.5 millimeter, and a length of 1.85 meters. An inlet tube 18 was fittedonto the slotted tube 21, which was in turn connected to the mixing headoutlet 12.

A frothing cavity 19 in the form of a diverging diffuser/baffle 19 wasattached to the outside of the gate bar or the pressure equalizationchamber 21 so as to form a diverging enclosed path from the elongatedslot 17 to the rectangular outlet 20 of the diffuser/baffle. Thedimensions of the outlet aperture of frothing cavity were 1.85 meterswide by 0.2 meters high. The length of the diffuser baffle form of thefrothing cavity from the slot to the outlet aperture was 0.5 meter.

The slotted tube, with the diffuser/baffle, was fitted onto theslab-stock foam machine so that the outlet aperture 20 was just abovethe bottom paper 2 and facing down the conveyor 1 in the direction ofproduction as shown by arrow P.

The gate bar 60 could have the configuration shown in FIGS. 6 and 7. InFIG. 6 the bar had a rectangular outer shape as well as a rectangularcross-sectional inner chamber 62. A series of elongated slots, as areshown at 64, 66 and 68, for example, could be used to provide thedesired outlet from the gate bar 60 and the desired pressure drop. InFIG. 7 the gate bar 70 is provided with a circular cross-sectionedinterior chamber 72 from which a series of tubular outlet apertures, asshown at 74, 76 and 78, for example, axially extend in the flowdirection to provide the desired openings and pressure drop.

The slab-stock foam machine was run in the same manner as for Example 2.The pressure in the tank 5 was again 6 bar as shown by pressure gauge14; the pressure in the mixing chamber was about 18 bar.

We have found that the pressure drop in the gate bar, or the pressureequalizing chamber 21 should be a significant part of the total pressuredrop from the mixing chamber onwards.

This time, the foam emerged from the rectangular aperture outlet 20 as asmooth froth. There was no evidence of large bubbles being present inthe expanding foam. The pressure indicated by the pressure gauge 12' onthe mixing head outlet tube 12 was 6 bar.

Foam expansion was completed at a distance shorter than 7 meters fromthe outlet aperture of the outlet 20. In consequence of the highviscosity of the froth, according to the invention it is possible toreduce the velocity of the conveyor and to dramatically reduce theoverall length of the line and therefore of the entire plant incomparison to a conventional one, without causing underrunning problems.

When the foam block had cured, it was found to have a height of 1 meter.The density of a sample of foam cut from the block was found to have adensity of 17 kg/m³. The cell structure was fine and contained no largevoids. The foam quality was judged to be commercially acceptable.

Example 4

According to the invention the same chemical recipe was used in Example3, with 6 bar pressure in the tank 5 containing the polyol blend/carbondioxide mixture, in pressure gauge 12'.

In this case, however, the outputs of the pumps 8, 9 and 10 were reducedto one fifth of the outputs used in Example 3, as shown in Table 1 andthe slot 17 reduced to a height of 0.4 mm.

The conveyors 1 and 21 were both run at a surface speed of 1meter/minute.

The expanding foam 16 in this case reached full expansion at a distanceof about 1.2 meters from the outlet 20.

The height of the cured foam block was 1 meter. The density of a sampleof foam cut from the block was measured to be 17 kg/m³. The cellstructure was fine and similar to the foam made in Example 3. Itcontained no large voids. The quality was judged to be commerciallyacceptable.

The examples 3 and 4 in this specification clearly demonstrate theefficiency of the process according to the present invention bycontrolling the pressure drop and the frothing of the mixture incontinuous production of polymeric foam material.

Example 5

The same chemical recipe has been used in the Example 4, except for theCO₂ content that has been increased as described below. The embodimenthas been modified in order to pump the liquid CO₂ in a continuous wayrather than pre-mixing it in the tank with the polyol. A pump 54, shownin FIG. 4, has been added in order to mix the liquid CO₂ from the tank55 in the polyol stream with the aid of a static mixer 53. A pressurereducing valve 56 has been introduced in order to assure that thepressure in the polyol line is maintained that will keep the CO₂ in aliquid state at the working temperature before the static mixer 53. Theliquid CO₂ has been pumped in the polyol stream at an output tocorrespond to a weight ratio of 4% (CO₂ on polyol). The liquid CO₂ couldbe pumped as well in the isocyanate stream. The slot 17 has been furtherreduced in height to about 0.3 mm. The pressure gauge 12' indicated apressure of 15 bars. All the other parameters were maintained as inExample 4.

The height of the cures block was 1.2 meters and the density was 14kg/m³. The cell structure was good.

Experimental tests conducted for a long time demonstrated the realpossibility to use CO₂ as primary blowing agent in an effective manner,having a smooth and homogeneous frothing of the reactant mixture on themoving substrate, as well as the foaming of the material while runningon the conveyor at a comparatively low velocity, thus resulting in amachine of substantially reduced length and output in respect ofmachines which make use of a conventional process or frothing technique.

In conventional mechanical mixing of flexible polyurethane foams, it iswell known that it is necessary to add small amounts of nucleating gasto the liquid reactants during mixing. The purpose of the nucleating gasis to provide nucleation sites for cell formation at the start offoaming. Typically, nucleating gas such as air or nitrogen would beadded at a rate of 0.3 to 3N liters per minute for a mixing throughputof 100 kg/minute of chemical reactants.

According to the invention we have also found that when liquid carbondioxide is introduced into the liquid reactants as an auxiliary blowingagent, it is still advisable to add a nucleating gas. The gas, as wouldbe expected, must be introduced into the mix at a sufficiently highpressure to overcome the pressure in the mixer. This pressure, aspreviously mentioned, can be about 5 to 18 bar.

We have further found that the quantity of nucleating gas can beconsiderably higher than in the case of conventional mechanical mixingof flexible polyurethane foam. At a pressure of 5-18 bar in the mixinghead and a chemical throughput of 100 kg/minute, we have found itpossible to add nucleating gas (eg. nitrogen) at the rate of 10-40Nliters/minute. If the lower addition rates typical of conventionalmechanical mixing are used, the foam cell structure is very coarse, oflow porosity and the foam product is not of commercial quality.

With reference now to FIGS. 4 and 5 an alternative form of the apparatusand the frothing device are shown.

According to the example of FIG. 4, in accordance with the presentinvention, the liquid CO₂ contained in the tank 55, by means of ametering pump 54, is directly injected in the flow of polyol fed fromthe tank 5 as in the example of FIG. 1. More precisely the liquid CO₂ isfed into the flow of polyol upstream a static mixer 53 connected to thehigh pressure mixer 11 for the polyurethane components by means of apressure-reducing valve 56. The valve 56 serves to ensure that thepressure in the polyol line be such to maintain the CO₂ in the liquidstate, at the working temperature. The liquid CO₂ could be differentlyintroduced into the flow of isocyanate fed from tank 6, in FIG. 1.

Reference 13 in FIG. 4 indicates an alternative embodiment of thepressure equalising and frothing device for the polyurethane mixture,shown in detail in FIG. 5. The tube 18 in FIG. 5, still comes from themixing chamber 11, as in the embodiments shown in FIGS. 1-3 and thepressure euqalization chamber or gate bar 21 is employed. Here thechamber 21 has a rectangular cross-sectional shape and shapes other thancircular or rectangular could be used. The gate bar also includes apressure reduction aperture 17. Aperture 17 opens into a modifiedfrothing cavity, generally indicated at 40. Frothing cavity 40 iscomprised of a top wall 42 that is attached to the gate bar and extendsoutwardly for a distance of about 10 mm. Wall 42 terminates at wall 44positioned at an angle of about 90° with respect to wall 42. Wall 44extends downwardly for about 30 mm and extends past the outlet of slot17. Accordingly, the emerging frothing mixture will intersect walls 42and 44 and have its direction turned 90° relative to the flow throughslot 17. We have found that ending the frothing cavity at this point,that is at the end of wall 44, and a portion of the rear wall 48, canproduce results that are satisfactory for controlling frothing and forproviding suitable back pressure on mixing chamber 11. We prefer,however, to continue the frothing cavity by continuing rear wall 48 fora distance of about 40 mm, by having a bottom wall 50 extend away fromrear wall 48, at an angle of about 90°, for a distance of about 50 mm.and by having top wall 46 extend from wall 44 at an angle of 90° and fora distance of about 40 mm. Walls 46 and 50 are positioned to besubstantially parallel although they could diverge at a slight angle ofabout 10° to about 20°.

By employing flow diverting walls 46 and 50 the frothing mixture makesanother 90° turn, this time relative to the direction the frothingmixture is flowing by reason of walls 42, 44 and the top portion of rearwall 48. Passage of the frothing mixture through the frothing cavityallows frothing to begin and occur, initially, under pressure controlledconditions. Passage by the frothing mixture through the frothing cavityhelps the initial frothing process to develop without the turbulenceassociated with direct injection systems. The resulting froth emerges ina smooth flowing manner from the outlet and produces a smooth, freeflowing transition onto the moving substrate in non-turbulent conditionas a creamed froth. Distribution is enhanced and the further transitionfrom frothing to reaction of the chemical ingredients, and foam growthas in generally indicated at 52, also occurs smoothly and morecompletely.

As FIG. 3 shows, the frothing cavity can be used with the movingcontinuous paper sheet and, where desirable, can also be used with thetop side paper sheet 22.

It is however evident that different or equivalent solutions arepossible, in respect to the examples previously described, withoutdeparting from the innovative principles of the present invention asclaimed.

What is claimed is:
 1. A system suitable for the continuous productionof polymeric slab-stock foam on a moving substrate comprising:a mixingdevice for mixing reactive chemical components and liquid CO₂ blowingagent under pressure, a feed line for carrying the mixture to a frothingdevice comprising a pressure equalizing chamber provided with a pressuredrop zone dimensioned to maintain back pressure on the upstream mixtureto keep the liquid CO₂ in a liquid state and to initiate frothing underpressure controlled conditions to avoid turbulent evaporation of theblowing agent upon discharge of the mixture from the pressure drop zone,and a frothing cavity having an outlet aperture extending transverselyacross the moving direction of the substrate and forming the dischargedmixture into a progressively non-reactive expanding frothing material byprogressively releasing the CO₂ in the frothing material as the frothingmaterial flows along the frothing cavity.
 2. A system for continuouslyproducing low density, flexible polyurethane slab-stock foam on a movingsubstrate comprising:a mixing device for mixing reactive chemicalcomponents and liquid CO₂ as a blowing agent under pressure; a feed linefor carrying the mixture to a frothing device comprising a pressureequalizing device provided with a pressure-drop zone dimensioned tomaintain back pressure on the upstream mixture to keep the liquid CO₂ ina liquid state and to initiate frothing under pressure controlledconditions to avoid turbulent evaporation of the blowing agent upondischarge of the mixture from the pressure drop zone; and a walledfrothing cavity extending from said pressure equalizing device, saidwalled frothing cavity being defined by walls that confine and permitthe discharged mixture to develop into a progressively non-reactiveexpanding frothing mixture by progressively releasing the CO₂ in thefrothing material as the frothing material flows along the frothingcavity, said frothing cavity terminating at an outlet aperture fordistributing an expanding frothing mixture onto and in the movingdirection of the substrate.
 3. A system according to claim 1 whereinsaid outlet aperture has a cross-sectional area greater than thecross-sectional area of said pressure drop zone.
 4. A system as in claim2 wherein said pressure-drop zone comprises an elongated slot.
 5. Asystem as in claim 2 wherein said pressure-drop zone comprises a seriesof elongated slots.
 6. A system as in claim 2 wherein said pressure-dropzone comprises a series of tubular outlet apertures axially extending inthe direction of the flow.
 7. A system as in claim 2 wherein saidpressure-drop zone comprises at least one slot that axially extends andis elongated in the direction of the flow of material therethrough.
 8. Asystem as in claim 2, wherein said frothing cavity is defined byinterconnected flow diverting wall means.
 9. A system as in claim 2wherein said walled frothing cavity includes walls that diverge in thedirection of flow.
 10. A system as in claim 2 wherein said pressureequalizing device comprises an elongated chamber.
 11. A system as inclaim 2, wherein said frothing cavity includes at least onesubstantially right angle turn downstream from said pressure drop zone.12. A system as in claim 2 wherein the frothing cavity is proportionedin its cross-sectional dimensions, in the direction of flow, toprogressively release the frothing mixture.
 13. A system as in claim 2wherein the pressure drop zone extends tranversally relative to thesubstrate, and wherein said outlet aperture has a greatercross-sectional area than said pressure-drop zone.
 14. A flexible foamslab-stock plant comprised of a pressurized chemical mixing station formixing polyurethane chemicals together with liquid CO₂ as a low boilingblowing agent maintained in a liquid state to produce a polyurethanefoam material, a discharge assembly connected to said chemical mixingstation for receiving the mixed polyurethane chemicals and fordeveloping the mixture into a homogeneous progressively pre-expandingnon-reactive froth material and depositing an expanding non-reactivefroth material onto a substrate moving along a fall plate, and aconveyor assembly positioned relative to said fall plate for receivingthe froth material discharged from said fall plate, said conveyor beingoperated at a rate of about 1 to 5 meters per minute so that the mixedpolyurethane chemicals can react with full block height being achievedwithin a range of about 1 to 8 meters from the deposit point.